A low carbon resulfurized and leaded free cutting steel, in which lead (Pb) and sulfur (S) are added as the free cutting elements to a low carbon steel for imparting a free-cutting capability to the steel, is known as a low carbon free cutting steel. However, there is a requirement for suppressing the use of Pb, which is used as one of the free cutting elements, in view of the earth environmental problem.
Such being the situation, Japanese Patent Disclosure (Kokai) No. 9-25539 (hereinafter referred to as “prior art 1”) discloses a free cutting microalloyed steel without quenching and tempering to which Pb is not added. In this case, Nd is added to the steel for promoting the finely dispersed precipitation of MnS. Japanese Patent Disclosure No. 2000-160284 (hereinafter referred to as “prior art 2”) also discloses a free cutting steel to which Pb is not added. In this case, a large amount of S is added to the steel so as to increase the amount of the sulfide, and the form of the sulfide is controlled by oxygen. Further, Japanese Patent Publication (Kokoku) No. 2-6824 (hereinafter referred to as “prior art 3”) discloses a free cutting steel, in which Cr having a reactivity with S to form a compound higher than that of Mn is added to the steel so as to form CrS in place of MnS, thereby improving the free-cutting capability.
However, prior art 1 is directed to a microalloyed steel containing 0.2 to 0.6% of C without quenching and tempering. In addition, a special element of Nd is used in prior art. It follows that it is impossible to comply sufficiently with the requirement for the cost reduction. Also, a large amount of S is added to the steel in prior art 2, with the result that the hot ductility of the steel tends to be lowered. Further, prior art 3 necessitates the addition of a large amount reaching 3.5 to 5.9% of costly Cr, resulting in failure to comply sufficiently with the requirement for the cost reduction. In addition, formation of a large amount of CrS as in prior art 3 is disadvantageous because the difficulty accompanying the smelting of the material is increased by the presence of a large amount of CrS.
There is a strong requirement for the further improvement in the machinability of the low carbon resulfurized and leaded free cutting steel in view of the reduction in the machining cost.
In compliance with the requirement, Japanese Patent Publication (Kokoku) No. 1-32302 B2 (hereinafter referred to as “prior art 4”) discloses a free cutting steel, in which a relatively large amount of S is added to the steel so as to increase the amount of the sulfide, and the form of the sulfide is controlled by Te, and the oxygen amount is suppressed to 0.0030% or less so as to decrease the number of alumina clusters, thereby improving the machinability of the free cutting steel. Also, Japanese Patent Disclosure No. 1-309946 (hereinafter referred to as “prior art 5”) discloses a free cutting steel, in which a relatively large amount of S is added to the steel so as to increase the amount of the sulfide, and a free cutting element of Pb is added to the steel so as to improve the machinability of the free cutting steel. Prior art 5 also teaches that the oxygen amount is suppressed to 0.008% or less for preventing the streak flaw caused by the gigantic oxide.
In each of prior arts 4 and 5, however, the form of the sulfide which effective for improving the machinability of the free cutting steel cannot be controlled sufficiently because the oxygen content of the steel is low, with the result that an elongated sulfide comes to be present in the steel. It follows that the free cutting steel is incapable of producing a sufficient effect of improving the machinability of the free cutting steel. Also, as described previously, the free cutting steel of prior art 2 is excellent in machinability because the form of a large amount of the sulfide is controlled by oxygen. However, the hot ductility of the free cutting steel tends to be lowered because a large amount of S is added to the steel.
On the other hand, the resulfurized and resulfurized and leaded free cutting steels contain in general a large amount of oxygen in order to control the form of the sulfide which is effective for improving the machinability of the free cutting steel. However, since all the oxygen does not dissolve in the sulfide, it is unavoidable for a gigantic oxide to be formed so as to cause the streak flaw, thereby giving rise to a serious defect in the processed article.
In prior art 5, the oxygen content of steel is suppressed to 0.008% or less in order to avoid generation of the streak flaw. In prior art 2, the required amount of oxygen is decreased by increasing the addition amount of S. Further, in prior art 1, the required amount of oxygen is decreased by using Nd as a free cutting element.
In prior art 5, however, the oxygen amount is simply decreased, though the oxygen amount is limited to 0.008% or less. Therefore, the form of the sulfide cannot be sufficiently controlled, as desired, with the result that an elongated sulfide comes to be present in the steel. It follows that the free cutting steel disclosed in prior art 5 cannot be said to be satisfactory in terms of the machinability. Also, concerning the free cutting steel disclosed in prior art 2, the reduction in the hot ductility caused by S is worried about as pointed out previously. Further, in prior art 1, as described above, there is a problem that it is difficult to reduce the cost.